Computer graphic and video projection devices have become commonplace and are in widespread use. Millions of business users are already using projection devices, and it is anticipated that additional millions of units will be used to provide entertainment viewing in the residential market. All so-called Front Projection Systems consist of a projection device that emits light, and a projection surface upon which this emitted light is reflected so that it may be viewed by the audience. Except in very unusual cases, the projection device, and the projection surface are two separate objects that may be oriented differently with respect to each other.
FIG. 1 is a schematic diagram of a prior art projection system. As shown, a projection device 5 is often mounted on a tabletop, and the surface of a nearby wall is used as a projection surface 7. Since the desired direction of the projected image for optimum viewing is centered at a point above the table, the projection device 5 is generally mounted on the table top in such a fashion that the axis of projection is inclined with respect to the plane of a flat table top as shown. The axis of projection is therefore not vertically perpendicular (normal) to the projection surface 7 as implemented by the wall. This off-axis vertical projection causes the top of the image, which is at a greater distance from the projection device than the bottom of the image, to subtend a larger portion of the projection surface than the bottom of the image in comparison to the input image as shown in FIGS. 2A and 2B resulting in vertical keystone distortion in the image. This type of visual distortion of the image commonly referred to in the video products industry as keystone distortion, because the trapezoidal shape of the resultant image is reminiscent of a keystone found in an arch.
FIGS. 3A to 3D are graphical representations showing the effect of a prior art vertical keystone correction method that uses pre-compensation techniques to achieve keystone correction in the vertical dimension. The conventional technique used to correct keystone distortion (keystone effect as shown in FIG. 3C) pre-compensates for keystone distortion by proportionately shrinking the lines at the top of the image, and expanding the lines at the bottom of the image as shown in FIG. 3B. The image in FIG. 3A is reproduced in dotted outline in FIG. 3B to show the relationship between the respective images. Typically the user adjusts the pre-compensation device with a slider control that increases or decreases the amount of keystone correction. The user's subjective impression that the keystone distortion has been neutralized (see FIG. 3D) is a critical part of the quality of the keystone correction. The image of FIG. 3C is reproduced in dotted outline in FIG. 3D to show the relationship between the respective images. Since there is only a single degree of freedom in a system that does keystone correction in the single vertical dimension (i.e. in the vertical direction), it is a relatively simple thing for a user to judge when keystone distortion had been properly corrected. However, this kind of manual keystone correction requires that the user be aware of keystone distortion, be aware that a keystone correction method exists, and understand or immediately become familiar with the projection device's keystone correction mechanism, and then properly perform keystone correction.
FIG. 4 is a schematic diagram illustrating the prior art inclination sensor method of establishing a zero reference and projection direction, with respect to the projection screen, for vertical keystone correction. Since the amount of keystone correction required in a system is only dependent on the angle of the axis of projection with respect to the normal vector of the projection surface, keystone correction may be automatically and optimally applied without user intervention as long as this angle is known. Systems which perform automatic keystone correction in only the vertical direction can utilize an inclination sensor to determine the angle of the axis of projection with respect to a zero reference in the vertical dimension. The zero reference in the vertical dimension for a typical vertical mounted projection screen is taken to be the vector of acceleration due to gravity. One possible method, for performing this operation is to affix an accelerometer 6 to the same surface that mounts the projection lens 9 as shown in FIG. 4. The accelerometer 6 thus measures the angle of the axis of projection with respect to acceleration due to gravity. The normal vector of the projection surface is then assumed to be always perpendicular to this zero reference, which is a generally valid assumption for a projection surface consisting of a wall and/or screens mounted on a wall.